Happiness in the sense of demeanour can be detrimental to insight, it is linked to the 'equanimity of unknowing' of the ordinary run-of-the-mill person. The response to most situations (which invariably involve defilements), requires a serious attitude. The Buddha regularly spoke of heedfulness as the required demeanour ( SN 3:17, Dhammapada chap. 2, Buddha's final instructions DN 16). In this sense in every event there is the opportunity to take a step towards either ignorance or wisdom. These decisions are what cumulatively contribute to a successful practice. Knowing and experiencing this, one tries to act skilfully in every situation.

"There is the case where an uninstructed, run-of-the-mill person — who has no regard for noble ones, is not well-versed or disciplined in their Dhamma; who has no regard for men of integrity, is not well-versed or disciplined in their Dhamma — does not discern what ideas are fit for attention or what ideas are unfit for attention. This being so, he does not attend to ideas fit for attention and attends [instead] to ideas unfit for attention."---MN 2

"The mode of thinking based on openness rejects duality as a product of discrimination and deluded concepts. It tacitly presupposes that existence as such is ultimately benign; that beyond our deluded concepts, the rich and vivid diversity of forms has a single taste, a taste that is sweet. In contrast, the attitude of heedfulness is grounded upon the view that existence is textured through and through by dualities that are profound and inescapably real. The world bears testimony to this vision in the contrast between the charming, delightful surfaces of things and their underlying hollowness and inadequacy; our minds bear testimony in the ongoing contest between the wholesome mental factors and the unwholesome ones, between the upward urge for purification and the downward pull of the defilements. That this duality is not trivial is seen by the consequences: the one leads to Nibbana, the state of deliverance, the Deathless, while the other leads back into the round of repeated birth, samsara, which is also the realm of Mara, the Lord of Death."---"A Note on Openness", Bikkhu Bodhi.

Duality in nature:
"When a snake's tongue flicks out, the two tines of the fork spread as wide as they can. ... Like your two ears help you identify which direction a sound comes from, the two tines of a snake's tongue tell the snake whether its prey ran left or right."

I would say 100%, and I don't think it's boasting, nor an insupportable claim to arahantship. I'm still treading the Path as all (most?) of you are and I have all the imaginable cankers. Furthermore, I have the impression that anger, fear, irritation or lust will not be totally eradicated in this present life, no matter how hard I try.

So why 100% of happiness? Because I've reached a point where, even when I'm angry, fearful, greedy or depressed, I know there is some underlying happiness. I know these are impermanent, suffering states, shallow currents of the heart that I don't need to identify with. I know, deep within, that I can transform them into compassion, equanimity, contentment and bliss. And I know I have the tools to do it, even if I don’t always succeed. That's the greatest gift the Buddhadhamma has given to me; not that it's not possible in other religions or philosophies, but I came in touch with it through the Buddhadhamma, which seems to me the clearest path. Weird as it may sound, even when I was through what one would call ‘depression’ I felt both depressed and somewhat happy. When you realize this, you feel that there is nothing whatsoever that might hurt you. Even if you are still far from immortality (amata), you feel you are invincible. Of course you are not, but happiness is all about feelings, isn’t it?

I believe the joy of having the Buddhadhamma should permeate our lives if we make use of it. People from countries like Myanmar or Cambodia are amongst the happiest folks I've met. Most of them don't practice in the monastic or meditational way, but their lives are in some way embedded into Dhamma; at least as long as they are untouched by rapidly-growing consumerism, sensual entertainment, religious fetishism and other modern-day deviances.

We all can achieve 100% 'happiness' as inner kindness and contentment, but very few will get even 50% of 'happiness' as delight, elation or euphoria: who would want to?

Yes, I am talking about pleasant feeling, Unpleasant feeling, and neutral feelings.
Now I think, natural feelings cannot be included in the happiness scale.
Hence what I am trying to measure here is the Pithy and Sukha.

For the deeper and final reasoning why the Buddha recommends an attitude of heedfulness, we have to go to the Abhidhamma:

“…the external universe, according to the Abhidhamma, is an outer reflection of the internal cosmos of mind, registering in concrete manifest form the subtle gradations in states of consciousness. This does not mean that the Abhidhamma reduces the outer world to a dimension of mind in the manner of philosophical idealism. The outer world is quite real and possesses objective existence. The outer world is always a world apprehended by consciousness, and the type of consciousness determines the nature of the world that appears. Consciousness and the world are mutually dependent and inextricably connected to such an extent that the hierarchical structure of the realms of existence exactly reproduces and corresponds to the hierarchical structure of consciousness.

Because of this correspondence, each of the two, the objective hierarchy of existence and the inner gradation of consciousness, provides the key to understanding the other. The reason why a living being is reborn into a particular realm is because he has generated, in a previous life, the kamma or volitional force of consciousness that leads to the rebirth into that realm, and thus the final analysis all the realms of activity of existence are formed, fashioned, and sustained by the mental activity of living beings. At the same time these realms provide the stage for consciousness to continue its evolution in a new personality and under a fresh set of circumstances.” “A Comprehensive Manual of Abhidhamma” p 188, Bikkhu Bodhi.

The uninstructed run-of-the-mill person takes the world to be there for their benefit and exploitation, and consequently has an attitude of either competitiveness or well-being, depending if they are working or recreating in it. Both these attitudes are destructive, the former proven externally by the environmental crisis and the latter internally by exposing the individual to the ravages of greed and aversion. Because the world is a product of our own mind, it simply reflects our current spiritual state and manifests our defilements, and an attitude of complacency towards it will not result in any advancement. Heedfulness is required to be alert to the opportunities in each situation, where there is a choice to be made either to perpetuate ignorance or take a step towards nibbana. By utilizing dhamma knowledge to step forward towards nibbana, we place ourselves as masters of the world, the true position, but it requires a revolution in conceptualisation of what the world is.

Now I think your happiness is related to your knowledge of Buddhism and how you practice it.
If you are unhappy (Dukkha) means you do not have a good understanding of Buddhism.
For many of us it is not easy to measure the happiness as a percentage.
As a guide say your are awake 16 hours and if you are happy for 8 hours, then you are happy 50% of the time.
Revise your score now. I kept mine unchanged.

Now I think your happiness is related to your knowledge of Buddhism and how you practice it.
If you are unhappy (Dukkha) means you do not have a good understanding of Buddhism.
For many of us it is not easy to measure the happiness as a percentage.
As a guide say your are awake 16 hours and if you are happy for 8 hours, then you are happy 50% of the time.
Revise your score now. I kept mine unchanged.

If I look at my Buddhist/spiritual self I am really happy .. and it shows on my face .. say 80% at least

But if I look at my conventional self (the one which is supposed to own things .. a car, a house, a good stereo system, a solid health insurance plan) I am really unhappy .. say no more than 20% happy .. I have not been able to accomplish anything on my bucket list

e.g. if I am by myself watching Breaking Bad (for the umpteenth time) or listening to a Dhammatalk on YT and munching peanuts .. I am near 90% happy .. but if the bell rings and a guest arrives .. I become acutely aware of may lack of material possessions and become 20% happy

Which happiness do you want me to report?

To put it differently I have a very happy spirit inside a very sad mind.

I think you are in the right track.
Use the following instead.
How many hours per day are you happy when you awake or half sleep?

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Within the Buddhist sutras, dukkha is divided in three categories:
Dukkha-dukkha, the dukkha of painful experiences. This includes the physical and mental sufferings of birth, aging, illness, dying; distress from what is not desirable.
Viparinama-dukkha, the dukkha of the changing nature of all things. This includes the frustration of not getting what you want.
Sankhara-dukkha, the dukkha of conditioned experience. This includes "a basic unsatisfactoriness pervading all existence, all forms of life, because all forms of life are changing, impermanent and without any inner core or substance."[web 1] On this level, the term indicates a lack of satisfaction, a sense that things never measure up to our expectations or standards

Now I think your happiness is related to your knowledge of Buddhism and how you practice it.
If you are unhappy (Dukkha) means you do not have a good understanding of Buddhism.
For many of us it is not easy to measure the happiness as a percentage.
As a guide say your are awake 16 hours and if you are happy for 8 hours, then you are happy 50% of the time.
Revise your score now. I kept mine unchanged.

But if I look at my conventional self (the one which is supposed to own things .. a car, a house, a good stereo system, a solid health insurance plan) I am really unhappy .. say no more than 20% happy .. I have not been able to accomplish anything on my bucket list

e.g. if I am by myself watching Breaking Bad (for the umpteenth time) or listening to a Dhammatalk on YT and munching peanuts .. I am near 90% happy

I find that the opposite makes me happy. I will give you a word of advice that even if you obtained all the items you listed, you might find yourself still around the 20% mark.

I recently bought a new (to me) car, the third one within the last year. I tried to justify it, of course, but in the end switching to another car was not absolutely necessary. Each time, I became less happy because I was searching for a "perfect" car that does not exist.

After you obtain everything on your list, it is tempting to think that you will wanting nothing else. But a new list of wants will quickly be created, and before you know it, you are at 20% again.

I voted for 80%. For whatever reason, I have felt a lot of happiness (not pleasure) recently. Walking home from work peacefully, realizing that I have nothing to worry about, meditating, selling or giving away items that I have not used in the past year (which, by the way, is a good rule of thumb for letting go of physical possessions when they are no longer useful).

After writing down my priorities in life, one of which is happiness, I asked myself how I should act if these are indeed my priorities. I began to notice actions, behaviors, and habits that reduced my happiness and long-term contentment. As a result, I have cut out a lot from my life, including consumption of news (and almost all other junk on the web), being with unwholesome people, and engrossing myself in entertainments. This has freed up a lot of time for reflection, meditation, and exploration, all of which increase my happiness.

"May the blessings of the Buddha, the Dhamma, and the Sangha always be firmly established in your hearts." ―Ajahn Chah

It is great to find a person who is happy 80% of the time.
Do you think that you will be able to maintain it rest of your life?
I am thinking about happiness in terms of Dukkha.
There is three type of Dukkha to be contemplated.

Now I think your happiness is related to your knowledge of Buddhism and how you practice it.
If you are unhappy (Dukkha) means you do not have a good understanding of Buddhism.
For many of us it is not easy to measure the happiness as a percentage.
As a guide say your are awake 16 hours and if you are happy for 8 hours, then you are happy 50% of the time.
Revise your score now. I kept mine unchanged.

But if I look at my conventional self (the one which is supposed to own things .. a car, a house, a good stereo system, a solid health insurance plan) I am really unhappy .. say no more than 20% happy .. I have not been able to accomplish anything on my bucket list

e.g. if I am by myself watching Breaking Bad (for the umpteenth time) or listening to a Dhammatalk on YT and munching peanuts .. I am near 90% happy

I find that the opposite makes me happy. I will give you a word of advice that even if you obtained all the items you listed, you might find yourself still around the 20% mark.

I recently bought a new (to me) car, the third one within the last year. I tried to justify it, of course, but in the end switching to another car was not absolutely necessary. Each time, I became less happy because I was searching for a "perfect" car that does not exist.

After you obtain everything on your list, it is tempting to think that you will wanting nothing else. But a new list of wants will quickly be created, and before you know it, you are at 20% again.

I voted for 80%. For whatever reason, I have felt a lot of happiness (not pleasure) recently. Walking home from work peacefully, realizing that I have nothing to worry about, meditating, selling or giving away items that I have not used in the past year (which, by the way, is a good rule of thumb for letting go of physical possessions when they are no longer useful).

After writing down my priorities in life, one of which is happiness, I asked myself how I should act if these are indeed my priorities. I began to notice actions, behaviors, and habits that reduced my happiness and long-term contentment. As a result, I have cut out a lot from my life, including consumption of news (and almost all other junk on the web), being with unwholesome people, and engrossing myself in entertainments. This has freed up a lot of time for reflection, meditation, and exploration, all of which increase my happiness.

In every situation there is a forked road, the practitioner can take either the path to indulgence, or make a choice of renunciation. Once it is understood through direct knowledge that renunciation fuels insight, then the alternative of renunciation becomes easier as they habituate themselves to the experience of selection and reward at a higher level. This is the work to be done.

It requires an effort of will on their part, as many of the obstacles blocking options of renunciation are imaginary and come from the conventions of ordinary life which have been ingrained since childhood: the practitioner feels obliged to act in habitual ways or face personal sanctions. Breaking out necessitates the point blank denial of of those promptings and instigating behaviour according with dhamma principles. This response transcends the cycles which the ordinary run-of-the-mill person is entrapped by, and are in contrast inspired and directed by personal faith in dhamma. It is necessary to act wilfully through faith at first without reward, confident in the insight which will later develop, and as motivation for action they immerse themselves in study of the internal unity of dhamma.